I was recently asked why a turbodiesel had begun emitting black smoke from the exhaust. The car had been regularly serviced (so I am told). The smoke was very slight under normal running about town, but disturbingly noticeable under heavy acceleration.
The car looked well maintained and the speedometer showed 100 000km. It seemed as if the exhaust gas recirculation system (EGR) was a likely suspect.
It helps to understand the operation of a diesel engine when such a fault arises. It relies on there being sufficient oxygen in the combustion chamber to allow complete burning of the fuel.
This is sometimes called the excess air principle.
When there is insufficient oxygen in the combustion chamber, unburned fuel enters the exhaust and burns there, hence the black smoke.
Another problem with the EGR system may be responsible for this. The main purpose of EGR is to reduce nitrous oxide emissions and normally the system computer regulates how much exhaust gas is recirculated and operates a solenoid valve to achieve this.
And here lies the weak link.
Like any mobile mechanical parts, this valve relies on free, smooth operation and can become sluggish or, in severe cases, stick due to the build-up of exhaust gas deposits. Smoke may also be caused by a mechanical problem such as a faulty injector, or by overloading and, or overfuelling the engine.
White smoke often appears when engine temperature is low and is caused by tiny droplets of unburned fuel. This rectifies itself when the engine reaches normal operating temperature.
Of course, diesel engines are far more sophisticated now and are no longer referred to as “dirty and smelly”, although a smoking engine may still produce an objectionable odour.
Fuel volatility is also worth a mention.
The distillation characteristics of a fuel describe its volatility. It must have low boiling components for easy cold starting and rapid warm-up. These are important considerations because too high or too low volatility could encourage carbon deposits, smoking and oil dilution due to vaporisation in the combustion chamber.
Handling fuel and ensuring that only absolutely clean fuel is used is important because many components of the fuel injection system have extremely tight clearances. This being so, any foreign material that somehow gets into the system may damage these finely finished parts and cause all sorts of problems and poor performance.
Diesels are tough and some have to work in appalling conditions, but clean fuel is essential if trouble is to be avoided. - Star Motoring